We shall continue our study of the mechanism of Escherichia coli topoisomerases. There are three aspects to the program. First, we will further analyze the binding of DNA to DNA gyrase. Gyrase recognizes particular sites in DNA. Part of this recognition mechanism is a nonanucleotide consensus sequence that we have studied previously. We have identified another sequence that appears to be 50-150 nucleotides on one side of the nonanucleotide. We hope to define it precisely by cloning. We will also continue our analysis of the sequences protected from nucleases by gyrase. Second, we are trying to define a unified mechanism for all topoisomerases; we postulate that type-1 topoisomerases differ from type-2 enzymes only in that an enzyme-bridged -strand break replaces the double strand interruption made by type-2 enzymes. We suggest that the swivel mechanism be abandoned. To explain catenation by type-2 enzymes, we postulate that they operate preferentially opposite a nick. This will be tested directly for several type-1 topoisomerases and the geometry of the cuts determined. Third, we are attempting to identify new topoisomerases in E. coli. We have evidence for several new enzymes. The enzymes will be purified and characterized and mutants sought in them.